In the literature, including the patent literature, various thickening agents derived from poly(meth)acrylic acid, optionally copolymerized with cross-linkers, are described. As an example, U.S. Pat. No. 2,798,053 reports thickeners known as CARBOPOL.RTM., obtained as powders by precipitation of the copolymer from different solvent systems and subsequent drying. However, the handling of volatile powders makes their use problematic and limits their corresponding employment; in addition, the aqueous dispersion of said products reaches high vicosities even at low concentrations, thus hampering their use in the form of stable suspensions.
In British Patent 870,994, the preparation of copolymers of methacrylic acid with alkyl acrylates is described, in which said copolymers are synthetized in concentrated (25-50% of solid content) and fluid aqueous emulsions.
In U.S. Pat. No. 4,138,381, the unsaturated carboxylic acids are copolymerized with alkyl acrylates and with esters between (meth)acrylic acid and polyethoxylated (5-80 moles of ethylene oxide) fatty alcohols (CO.sub.0-20), for the purpose of obtaining a better thickening power and a reduced sensitivity towards the salts present in the solution to be thickened. The polymerization described in this patent is carried out in glycol or in a 50% water-glycol mixture.
EP A1 0013836 reports the copolymerization in aqueous emulsion of (meth)acrylic acid with alkyl acrylates and an ester of (meth)acrylic acid with a (C.sub.8-30) alkyl, alkylaryl or polycyclic alkyl monoether of a polyethyleneglycol. The polymerization is optionally carried out in the presence of a cross-linking agent, which, however, is employed in quantities never higher than 1%, calculated on the total weight of the monomers. Products are obtained endowed with thickening properties which, in addition, possess an acceptable resistance towards the electrolytes which may be present in the variuos systems to be thickened.
Similarly, in EP A1 0109820 and EP B1 0217485, cross-linked copolymers are described, which are prepared by polymerization in aqueous emulsion and in which the cross-linking agent, when employed, is again present in quantities which are never higher than 1%, calculated on the total weight of the monomers. Also these copolymers seem to possess a reduced sensitivity towards electrolytes.
EP A1 0658579 reports the preparation of non cross-linked thickening agents deriving from the copolymerization of a monomeric system consisting of, among others, (meth)acrylic acid, an ester of (meth)acrylic acid and an associative monomer which is the sorbate of an O-alkyl-polyoxyethylene glycol.
Finally, in WO 96/35757, aqueous print pastes are described which are thickened by cross-linked copolymers having a particle size higher than 200 nm. These copolymers are obtained by polymerization in aqueous emulsion of a (meth)acrylic acid/(meth)acrylate/allyl-O-PEG-O-alkyl system in the presence of a cross-linking agent which is preferably employed in amounts not higher than 0.05%; it is also represented that, in order to obtain an aqueous print paste possessing improved color yield, the polymerization is advantageously carried out in the presence of ethoxylated alcohol phosphates as surfactants/emulsifiers.